KNa3(UO2)2(Si4O10)2(H2O)4, a new compound formed during vapor hydration of an actinide-bearing borosilicate waste glass

Burns, Peter C.; Olson, Rudolph A; Finch, Robert J.; Hanchar, John M.; Thibault, Yves

doi:10.1016/s0022-3115(99)00247-0

Cited by 92 publications

(56 citation statements)

References 12 publications

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“…Other, less common, minerals include uranosilite, (UO 2 ) 2 Si 7 O 15 (H 2 O) n (0 < n < 1), with a ratio of 3.5 (Walenta, 1974). Investigations of hydrothermal glass alteration and the synthesis of sodium-uranyl compounds (Plesko et al, 1992;Burns et al, 2000;Li and Burns, 2001), have resulted in solid compounds with a ratio of 4, and the EMP-determined weight-percent composition of the synthesized 4:1 compound KNa 3 (UO 2 ) 2 (Si 4 O 10 ) 2 (H 2 O) 4 (Burns et al, 2000) is included in Table 1 for comparison with our unknown. Although the weight percent data for the unknown fracture-fill mineral were comparable to the results for KNa 3 (UO 2 ) 2 (Si 4 O 10 ) 2 (H 2 O) 4 (Table 1), this comparison was deceptive, and further evaluation suggested that it was not accurate.…”

Section: Discussionmentioning

confidence: 99%

Microscale controls on the fate of contaminant uranium in the vadose zone, Hanford Site, Washington

McKinley

Zachara

Liu

et al. 2006

Geochimica et Cosmochimica Acta

103

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An alkaline brine containing uranyl (UO 2 2+ ) leaked to the thick unsaturated zone at the Hanford Site. We examined samples from this zone at microscopic scale to determine the mode of uranium occurrence-microprecipitates of uranyl (UO 2 2+ ) silicate within lithic-clast microfractures-and constructed a conceptual model for its emplacement, which we tested using a model of reactive diffusion at that scale. The study was driven by the need to understand the heterogeneous distribution of uranium and the chemical processes that controlled it. X-ray and electron microprobe imaging showed that the uranium was associated with a minority of clasts, specifically granitic clasts occupying less than four percent of the sediment volume. XANES analysis at micron resolution showed the uranium to be hexavalent. The uranium was precipitated in microfractures as radiating clusters of uranyl silicates, and sorbed uranium was not observed on other surfaces. Compositional determinations of the 1-3 lm precipitates were difficult, but indicated a uranyl silicate. These observations suggested that uranyl was removed from pore waters by diffusion and precipitation in microfractures, where dissolved silica within the granite-equilibrated solution would cause supersaturation with respect to sodium boltwoodite. This hypothesis was tested using a reactive diffusion model operating at microscale. Conditions favoring precipitation were simulated to be transient, and driven by the compositional contrast between pore and fracture space. Pore-space conditions, including alkaline pH, were eventually imposed on the microfracture environment. However, conditions favoring precipitation were prolonged within the microfracture by reaction at the silicate mineral surface to buffer pH in a solubility limiting acidic state, and to replenish dissolved silica. During this time, uranyl was additionally removed to the fracture space by diffusion from pore space. Uranyl is effectively immobilized within the microfracture environment within the presently unsaturated Vadose Zone.

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Section: Discussionmentioning

confidence: 99%

Microscale controls on the fate of contaminant uranium in the vadose zone, Hanford Site, Washington

McKinley

Zachara

Liu

et al. 2006

Geochimica et Cosmochimica Acta

103

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“…[136,137] The smectite clays are always in contact with the unaltered glass while analcime is further removed [136,137,154] [156].…”

Section: -----------------Increasing Solution Ph ---------------------→mentioning

confidence: 99%

Durable Glass for Thousands of Years

Jantzen

Brown

Pickett

2010

Int J of Appl Glass Sci

180

156

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The durability of natural glasses on geological time scales and ancient glasses for thousands of years is well documented. The necessity to predict the durability of high level nuclear waste (HLW) glasses on extended time scales has led to various thermodynamic and kinetic approaches. Advances in the measurement of medium range order (MRO) in glasses has led to the understanding that the molecular structure of a glass, and thus the glass composition, controls the glass durability by establishing the distribution of ion exchange sites, hydrolysis sites, and the access of water to those sites. During the early stages of glass dissolution, a "gel" layer resembling a membrane forms through which ions exchange between the glass and the leachant. The hydrated gel layer exhibits acid/base properties which are manifested as the pH dependence of the thickness and nature of the gel layer. The gel layer ages into clay or zeolite minerals by Ostwald ripening. Zeolite mineral assemblages (higher pH and Al 3+ rich glasses) may cause the dissolution rate to increase which is undesirable for long-term performance of glass in the environment. Thermodynamic and structural approaches to the prediction of glass durability are compared versus Ostwald ripening.

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“…The Na(1), Na(2), Na(3), and Na(6) cations are each coordinated by six ligands arranged at the vertices of distorted octahedra, with 1Na}2 (: O, H O) bond lengths ranging from 2.419 to 2.477 A > . The Na (2) and Na(3) octahedra contain only O atoms, whereas the Na (1) octahedron contains "ve O atoms and one H O group, and the Na (6) octahedron has four O atoms and two H O groups. The Na(4) cation is coordinated by seven atoms of O.…”

Section: Cation Polyhedramentioning

confidence: 99%